1. Field of the Invention
The present invention relates to a ceramic multilayer substrate, and more particularly, to a ceramic multilayer substrate having a cavity.
2. Description of the Related Art
A ceramic multilayer substrate having a cavity structure has been used to increase the packaging density and to reduce the size and the profile of an electronic component.
A ceramic multilayer substrate having a cavity is formed by stacking a green ceramic layer having no opening and a green ceramic layer having an opening and firing them to form a cavity. In a ceramic multilayer substrate having a cavity, differences in the shrinkage of ceramic layers during firing tend to cause cracks at a bottom end of the cavity and deformation, such as warp or waviness, at a portion forming the bottom of the cavity or a side wall on the perimeter of the cavity. Various techniques have been proposed to prevent such failures.
For example, Japanese Unexamined Patent Application Publication No. 2003-60106 (Patent Document 1) discloses providing an interposing layer at a portion forming a side wall on the perimeter of a cavity. The interposing layer has a smaller shrinkage than the ceramic layers during firing. Japanese Unexamined Patent Application Publication No. 2002-164654 (Patent Document 2) discloses providing a stress relieving pad, which reduces a contraction stress generated at an interface between ceramic layers, in the vicinity of a bottom end of the internal surface of a cavity. Japanese Unexamined Patent Application Publication No. 2004-165247 (Patent Document 3) discloses embedding a pattern for retaining the shape, which is formed of a ceramic material having a minimum sintering temperature greater than that of a ceramic material for a green sheet for forming a substrate, along the vicinity of the corners of a cavity in a ceramic multilayer substrate.
Furthermore, Japanese Unexamined Patent Application Publication No. 10-173083 (Patent Document 4) discloses a cavity having a step that is deeper in the center than in the periphery of the cavity.
Furthermore, Japanese Unexamined Patent Application Publication No. 5-243700 (Patent Document 5) and Japanese Unexamined Patent Application Publication No. 2002-26528 (Patent Document 6) disclose lowering the minimum sintering temperature of a conductive paste for an internal electrode below the minimum sintering temperature of a ceramic green sheet.
When an interposing layer is provided to control the shrinkage of a ceramic layer during firing, a circuit provided in a ceramic multilayer substrate is subjected to stricter design restrictions depending on the position of the interposing layer. In addition, because the interposing layer differs in characteristics or shape from a ceramic green sheet forming a major portion of a substrate, an additional specific process is needed. Hence, it is difficult to reduce the manufacturing costs.
Similarly, since a stress relieving pad or a pattern for retaining the shape must have a specific shape or characteristic, it requires another specific process. Thus, it is difficult to reduce the manufacturing costs.